In relatively small displacement multi-cylinder internal combustion engines of the type having overhead valve gear, for example, 4-cylinder in-line engines, it is not uncommon for such engines to operate at high rotational speeds, for example, 4,000-6,000 RPM during a large percentage of the normal duty cycle. In this high RPM regimen, the inertial forces of the valve gear can become critical with respect to the valve spring closing force. In order to reduce the inertial forces of the valve gear, it is desirable to eliminate the push rod-type valve gear and utilize a direct-acting or cam-over-rocker arrangement. This arrangement eliminates the tappets and push rods between the camshaft and the valve rocker arms. However, in designing valve gear for a cam-over-rocker arrangement where the cam lobes contact the rocker arms directly, the usual technique, employed in push rod-type valve gear of providing lash adjustment in the tappet, is not available. Whereas, in the conventional push rod-type valve gear, the lash adjustment is usually provided in the form of a combination tappet-hydraulic valve lifter between the push rod and the camshaft. Where attempts have been made to utilize hydraulic lash adjusters for cam-overrocker valve gear, it has been found that the most compact arrangement is to provide the lash adjustment at a stationary pivot about which one end of the valve rocker is pivoted. However, where the lash adjusters operate from the pressurized engine oil, the force obtained from the oil pressure acting on the end of the adjuster plunger is increased by the mechanical leverage of the rocker and applied to the cam lobe at the rocker contact surface. This additional force results in the need for greater valve spring forces at high RPM.
Most prior art hydraulic lash adjusters are provided with a body retainer which operates to contain the plunger assembly within the body after assembly and prior to installation in a valve gear of an internal combustion engine. Typical body retainers are formed of stamped metal and have a central opening positioned over the open end of the body in register with the plunger assembly. The plunger assembly is provided with a shoulder which abuts the retainer. The body will typically include a groove on the exterior thereof and an overlying portion of the retainer is permanently plastically deformed for retention therewith.
An alternative approach has been the use of a snap or split ring action retainer which embraces the outer body surface.
Although widely commercially practiced, these approaches have a number of shortcomings. The metal retainers must be permanently deformed during assembly, a process requiring special tooling and process control to maintain acceptable tolerances. This form of mechanical attachment produces only point contact between the retainer and the body whereby the retaining forces are unequally distributed about the body. Such unequal force distribution can distort the body, imparing plunger movement and interfit within the mating head bore. Because assembly is typically affected at room temperature, the clamping forces can be lost or very substantially reduced at elevated temperatures found during normal engine operating conditions whereby the retainer can release from the body and interfere with valve train operation. Split ring type retainers likewise apply uneven clamping forces and are prone to fatigue and early catastrophic failure. A further disadvantage of the deformed metal type retainers is evident in lash adjuster designs employing a bulbous or enlarged end portion of the plunger assembly which abuts the rocker arm to reduce per unit surface loading. The deformed metal approach is wholly unacceptable for this application, leaving only the split ring approach with its above-mentioned shortcomings. A further disadvantage of prior art metal retainers arises from their physical size and the difficulty of adequately identifying a retainer prior to assembly in a lash adjuster. As is often the case, a manufacturer may produce many variations of a particular adjuster, the retainers of each which may be dimensionally distinct but visually indistinguishable. Problems arising from confusion of the identity of parts and misassembly is self-evident.